Zoom lens for projection and projection-type display device

Abstract

A zoom lens for projection includes negative first group G 1 , positive second group G 2 , positive third group G 3 , negative fourth group G 4 , positive fifth group G 5 , and positive sixth group G 6 , arranged from the magnification side of the zoom lens. The reduction side of the zoom lens is telecentric. When the magnification is changed, the first group G 1 and the sixth group G 6 are fixed, and the second through fifth groups G 2 through G 5 move. The fourth group G 4 is a negative lens having a concave surface facing the magnification side, and the fifth group G 5 includes at least two positive lenses and at least two negative lenses. Further, the following formula (1) is satisfied: −5.0≦ f 4/ fw ≦−1.2  (1), where f4: the focal length of the fourth group; and fw: the focal length of the entire system of the zoom lens at a wide angle end.

1. A zoom lens for projection comprising:
 a first lens group having a negative refractive power; 
 a second lens group having a positive refractive power; 
 a third lens group having a positive refractive power; 
 a fourth lens group having a negative refractive power; 
 a fifth lens group having a positive refractive power; and 
 a sixth lens group having a positive refractive power, which are arranged from a magnification side of the zoom lens, 
 wherein a reduction side of the zoom lens is telecentric, and 
 wherein when the magnification of the zoom lens is changed, the first lens group and the sixth lens group are fixed, and each of the second lens group, the third lens group, the fourth lens group and the fifth lens group moves along the optical axis of the zoom lens, and 
 wherein the fourth lens group is a negative lens having a concave surface facing the magnification side, and wherein the fifth lens group includes at least two positive lenses and at least two negative lenses, and 
 wherein the following formula (1) is satisfied:
   −5.0 ≦f 4/ fw≦− 1.2  (1),
 
 
 where f4: the focal length of the fourth lens group; and 
 fw: the focal length of the entire system of the zoom lens at a wide angle end. 
 
     
     
       2. A zoom lens for projection, as defined in  claim 1 , wherein the following formula (2) is satisfied:
   2.0 ≦Bf/fw≦ 5.0  (2),
 
 where Bf: back focus in air on the reduction side of the entire system of the zoom lens. 
 
     
     
       3. A zoom lens for projection, as defined in  claim 1 , wherein the following formula (3) is satisfied:
   1.5 ≦ft/fw   (3),
 
 where ft: the focal length of the entire system of the zoom lens at a telescopic end. 
 
     
     
       4. A zoom lens for projection, as defined in  claim 1 , wherein the following formula (4) is satisfied:
   55ν5  (4),
 
 where ν5: an average value of Abbe numbers of the positive lenses included in the fifth lens group. 
 
     
     
       5. A zoom lens for projection, as defined in  claim 1 , wherein the fifth lens group includes a negative lens, a positive lens, a negative lens and a positive lens that are arranged from the magnification side. 
     
     
       6. A zoom lens for projection, as defined in  claim 5 , wherein the negative lens, the positive lens, the negative lens, and the positive lens in the fifth lens group form two cemented lenses, each of which is composed of a negative lens and a positive lens. 
     
     
       7. A zoom lens for projection, as defined in  claim 1 , wherein the first lens group includes at least one aspheric surface. 
     
     
       8. A projection-type display device comprising:
 a light source; 
 a light bulb; 
 an illumination optical unit that guides a light beam output from the light source to the light bulb; and 
 a zoom lens for projection, as defined in  claim 1 , the reduction side of the zoom lens being telecentric, wherein the light beam output from the light source is modulated by the light bulb, and projected onto a screen by the zoom lens for projection.